The present invention relates to laser amplifiers, and more particularly to a solid-state laser incorporating multiple disk-shaped laser gain media (subapertures) placed adjacent to each other to fill an optical aperture of an AMA module.
Thermomechanical effects present a major challenge to scaling of a solid-state laser (SSL) to high-average power (HAP). In particular, optical distortions caused by transverse temperature gradients (i.e., perpendicular to laser beam axis) are known to degrade beam quality, which may render the beam useless for many important applications. A class of SSL known as xe2x80x9cactive mirror amplifierxe2x80x9d (AMA) originally disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971) has shown effective reduction of transverse temperature gradients and demonstrated generation of laser output with very good beam quality. See, for example, J. Abate et al., xe2x80x9cActive Mirror: A Large-Aperture Medium Repetition Rate Nd:Glass Amplifier,xe2x80x9d Appl. Opt. Vol. 20, no. 2, 351-361 (1981) and D. C. Brown et al., xe2x80x9cActive-Mirror Amplifier: Progress and Prospects,xe2x80x9d IEEE J. of Quant. Electr., vol. 17., no. 9, 1755-1765 (1981). In the classical AMA concept, a large aspect ratio, edge-suspended, Nd-Glass disk (or slab) several centimeters thick is pumped by flashlamps and liquid-cooled on the back face. However, this device is not suitable for operation at HAP because of poor heat removal and resulting thermo-mechanical distortion of the edge-suspended disk. Previous attempts to mitigate these problems and increase the average power output of AMA have been met with encouraging but limited results. In recent years, the AMA concept has been a revived in the form of a xe2x80x9cthin disk laserxe2x80x9d introduced by Brauch et al. in U.S. Pat. No. 5,553,088 (1996). The thin disk laser uses a gain medium disk several millimeters in diameter and 200-400 micrometers in thickness soldered to a heat sink. See, for example A. Giesen et al., xe2x80x9cScalable Concept For Diode-pumped High-power Lasers,xe2x80x9d Appl. Phys. B vol. 58, 365-372 (1994). A diode pumped Yb:YAG thin disk laser has demonstrated laser outputs approaching 1 kW average power and with beam quality around 12 times the diffraction limit. See for example, C. Stewen et al., xe2x80x9c1-kW CW Thin Disk Laser,xe2x80x9d IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650-657 (July/August 2000). Another variant of the thin disk laser can be found in L. Zapata et al., xe2x80x9cComposite Thin-disk Laser Scalable To 100 kW Average Power Output and Beyond,xe2x80x9d in Technical Digest from the Solid-State and Diode Laser Technology Review held in Albuquerque, N. Mex., Jun. 5-8, 2000.
The applicant""s first co-pending patent application Ser. No. 99/505,399 entitled xe2x80x9cActive Mirror Amplifier System and Method for a High-Average Power Laser Systemxe2x80x9d, which is hereby made a part hereof and incorporated herein by reference, discloses a new AMA concept suitable for operation at high-average power. This invention uses a large aperture laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 and 15 cm, mounted on a rigid, cooled substrate. Note that the disk thickness in this AMA concept is about 10 times less than in the classical AMA and about 10 times more than in the thin disk laser. The substrate contains a heat exchanger and microchannels on the surface facing the laser medium disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels. This novel approach permits thermal expansion of the laser medium disk in the transverse direction while maintaining a thermally loaded disk in a flat condition. The above-mentioned patent application Ser. No. 99/505,399 teaches two principal methods for providing pump radiation into the AMA disk, namely: 1) through the large (front or back) face of the disk, or 2) through the sides (edges) of the disk. AMA using the former method, which is often referred to as xe2x80x9cface pumping,xe2x80x9d is further elaborated, for example, in J. Vetrovec, xe2x80x9cActive mirror amplifier for high-average power,xe2x80x9d to be published in SPIE vol. 4270 (2000). FIG. 1 shows such a face-pumped AMA where pump radiation from a diode array is injected into the laser gain medium through an optically transparent substrate.
The applicant""s second co-pending patent application, entitled xe2x80x9cSide-Pumped Active Mirror Solid-State Laser for High-Average Powerxe2x80x9d, Docket No. 00-173 filed on Jan. 22, 2001, which is hereby made a part hereof and incorporated herein by reference, discloses a composite AMA wherein optical pump radiation is injected into the peripheral edge of a composite gain medium disk. Side-pumping takes advantage of the long absorption path (approximately the same dimension as disk diameter), which permits doping the disk with a reduced concentration of lasant ions and a corresponding reduction in pump radiation intensity. The composite gain medium is formed by bonding an undoped optical medium to the peripheral edges of the laser gain medium disk. This construction facilitates improved coupling between the source of optical pump radiation and the laser gain medium, as well as concentration of optical pump radiation, cooling of the peripheral edge of the laser gain medium disk, and providing a trap for amplified spontaneous emission (ASE). In that invention, sources of optical pump radiation are placed around the perimeter of the composite gain medium. Tapered ducts may be disposed between the sources of optical pump radiation and the composite gain medium for the purpose of concentrating optical pump radiation. With the proper choice of laser gain medium doping, pump source divergence and geometry, a uniform laser gain is achieved across large portions of the gain medium.
The teachings of the two above-mentioned co-pending patent applications of the Applicant provide numerous advantages over prior art SSL and allow generation of near diffraction limited laser output at very high-average power from a relatively small device. However, these co-pending patent applications disclose only AMA modules utilizing single monolithic laser gain medium covering the device optical aperture. The term xe2x80x9caperturexe2x80x9d as used herein is the one typically used in optics, namely: xe2x80x9cThe diameter of the objective of a telescope or other optical instrumentxe2x80x9d, as defined in McGraw-Hill Dictionary of Scientific and Technical Terms, 4th edition, published by McGraw-Hill, Inc.
To obtain higher average laser power from an AMA module, it is beneficial to increase the size of the optical aperture. However, the size of a single monolithic laser gain medium disk required to fill the optical aperture is limited by available fabrication technology. In particular, YAG crystal boules can be reliably grown only to about 5 cm diameter and GGG crystals to about 15 cm diameter. The difficulties, limitations, and cost of growing large crystals pertinent to the subject invention are discussed, for example, in D. Dawnes, xe2x80x9cNd:YAGxe2x80x94The Versatile High-power Solid-state Laser Crystal,xe2x80x9d published in Industrial Laser Review in March 1995.
Another consideration associated with using large AMA disks is a tradeoff between transverse dimensions of the disk and the producible laser gain. In particular, a larger size AMA disk can produce higher average laser power but to avoid excessive ASE losses, this must be done with much lower gain than a comparably smaller AMA disk. This can be a significant limitation when optimizing an ultrahigh-average power system, where a very large number of AMA stages with monolithic apertures would be needed to meet a particular output power and gain requirements for efficient operation of a laser resonator.
The present invention provides an apparatus and method for achieving improved performance in a solid-state laser. The solid-state laser of the present invention uses multiple disk-shaped laser gain media (subapertures) placed adjacent to each other to fill an optical aperture of an AMA module. The perimeter of each disk may be circular, elliptical, rectangular, polygonal, or formed by linear segments, or a combination of linear segments and curves. Boundaries of adjacent disks are chosen so that gaps between disks are minimized and disks efficiently cover most of the AMA optical aperture. In the preferred embodiment each laser gain medium is provided with optical coatings for operation in the active mirror configuration. Furthermore, each laser gain medium is pressure-clamped to a rigid, cooled substrate. The substrate has a plurality of internal passages leading up to its contact surface with the optical gain medium. A hydrostatic pressure-clamping effect is generated by maintaining the passages at a substantially lower pressure than the pressure of the atmosphere in which the solid-state laser is immersed. A laser gain medium constrained in this fashion can maintain a prescribed shape even when experiencing significant thermal load. The substrate may be common to all disks, or several disks, or individual to one or more disks.
Pumping of the laser gain media to laser transition is accomplished by providing optical pump radiation either to the front or back faces of the disks or to selected portions of the peripheral edges of the disks. The laser gain medium disks are preferably of composite construction formed by attaching undoped optical medium and/or ASE absorption cladding and/or ASE absorption coating to the peripheral edges of the laser gain medium disk. When an undoped optical medium is used, it is adapted for receiving optical pump radiation and transporting it into the laser gain medium. A preferred method of attaching the undoped optical medium to the laser gain medium is by optical contacting followed by heat treatment. ASE absorption cladding can be either diffusion bonded or adhesive bonded. Alternately, an ASE absorption coating can be used in lieu of cladding.
The use of a composite gain medium provides several advantages in the present invention. First, the undoped optical medium facilitates improved coupling between the source of optical pump radiation and the laser gain medium. Second, the undoped optical medium may additionally concentrate optical pump radiation by having a converging (i.e., tapered) profile and/or curved input surfaces. Third, the undoped optical medium cools the peripheral edge of the laser gain medium disk by drawing heat therefrom. Fourth, the undoped optical medium serves as a trap for ASE rays, thereby significantly reducing feedback to parasitic oscillations. ASE absorption cladding and ASE absorption coating also significantly reduce the feedback to parasitic oscillations.
In one preferred embodiment a plurality of sources of optical pump radiation are placed so as to inject pump radiation into the front or back faces of the laser gain medium disks. In the latter case, pump radiation is injected through the substrate on which the disks are mounted and which, for this purpose, is made of optically transparent material. Pump source intensity may be varied across the aperture to counter the effects of laser gain saturation and, thereby, produce a more uniform saturated laser gain. The cooling effect too may be correspondingly varied across the aperture to reduce transverse temperature gradients. In another preferred embodiment, sources of optical pump radiation are placed so as to inject pump radiation into selected perimeter edges of the laser gain media disks. Regardless of the method of injecting optical pump radiation, tapered ducts for concentration of optical pump radiation may be disposed between the sources and the laser gain medium. Such ducts can be either made of solid optical material or be constructed as hollow shells with reflecting internal surfaces, which can be either empty or filled with transparent liquid. An additional concentration of optical pump radiation can be achieved with either microlenses integrated into the optical pump sources and/or lenses placed in front of the emitting surface of the sources.
A cooling medium can be provided to a heat exchanger internal to the substrate and/or flowed through the passages on the substrate surface, thereby directly wetting the laser gain medium. Preferably, coolant fluid connections to the substrate are provided by pressure-balanced fluid transfer tubes permitting small axial and transverse movements. Such fluid transfer tubes isolate hydraulic pressure loads from the substrate and coolant supply so that alignment of the substrate will not be affected. In addition, the fluid transfer tubes balance the hydraulic forces caused by the coolant pressure so that the substrate will not have any significant load placed upon it to interfere with its operation.
The invention can be used as a building block for the construction of laser oscillators as well as laser amplifiers operating in a pulsed (storage) mode, continuous wave (cw) mode, and quasi-cw (long pulse) mode.